Importing C++ functions created with pybind11
So after some digging I found out, that this was caused by importlib.reload.
Blenders Reload Scripts-command only reloads the addon if the main file (in my case __init__.py) has been changed. (Also discussed here)
Since my addon consists out of several .py-files and I wanted everything to be reloaded if I hit the command, I used importlib.reload to manually force reloading.
I also used this technique on my C++-module, which you're not supposed to do!
Reloading C++-modules with importlib is not supported and may result in errors, just like in my case.
After removing this reloading instruction my C++-module works just fine in Blender. The only downside is, that I'll have to restart Blender, whenever I change my C++-module and want to see these changes in effect.